The black oxide coating appears on a variety of products – most commonly drill bits, pocket knives and other cutting accessories. What is Black Oxide Coating? In short, it coats the material through a chemical transformation rather than an electroplating process. According to Electrochemical Products, Inc., you can apply this type of cladding to steel, stainless steel, aluminum, cast iron, copper, brass, bronze, zinc, and other materials.
Table of contents
- Why Use Black Oxide
- Black oxide coating in a nutshell
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- Benefits of Black Oxide Coating
- Coating Limitations
- Apply black oxide coating using hot or cold process
- Post-treatment of black oxide coatings
Why Use Black Oxide
First, black oxide prevents oxidation. Second, it helps keep the edges of products like drills and certain tools sharp. It also reduces friction. Finally, a black oxide coating reduces light reflection. This makes them absolutely perfect (and vital) for a wide variety of UV and IR sensors and detectors.
You can find this process and material in a variety of applications and industries. These include automotive, sensors, photovoltaics – even watchmaking and applications involving the aerospace industry!
Black oxide coating in a nutshell
Black oxide coatings (also known as magnetite or Fe3O4 ) come in many forms and names. You could call it blackening or black passivation, or even gun bluing. Adding a black oxide coating refers to the use of a chemical conversion process to form a black iron oxide coating on any ferrous metal surface.
This is different from powder coating or titanium coating on metal surfaces. Instead, the process that produces a black oxide coating results from a chemical reaction between the iron sitting on the surface of the ferrous metal and a black oxide solution consisting of oxidizing salts.
Manufacturers are not simply stopping at the black oxide coating process, but tend to continue to treat the metal using a variety of methods. This may include additional processes to increase corrosion and wear resistance.
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As mentioned above, manufacturers use chemical conversion to create the black oxide coating directly on the material, rather than applying it like an electroplating process. The black oxide solution includes a salt that acts as an oxidizing agent. The solution reacts with the iron in the steel to form a coating on the surface.
On products such as blades, manufacturers coat the blades before adding the final edge. This reveals the silver edge on the brand new blade. Instead, the drill ends in a chemical bath. When new, you can actually see the coating on the cutting edge. Using this drill or sharpening it with Drill Doctor, the natural uncoated edge is clearly visible.
Benefits of Black Oxide Coating
The best drills use more expensive coatings or materials. Still, black oxide has its uses. Manufacturers choose it as an inexpensive way to improve steel compared to other coatings and mixtures. Two of the most popular benefits of this material include corrosion resistance and reduced friction. Both are relatively minor improvements, but it does elevate the product over bare steel.
Black oxide coatings also add little to the thickness of the steel they are applied to. For precision accessories like drills, adding as little as 5 to 10 millionths of an inch will help keep them sharp.
The black oxide coating also gives the product a nice tactical look. It has the sheen of raw steel, so some look shiny, while others have a non-reflective matte finish.
other benefits
- better lubrication between components
- anti-wear surface
- Better adhesion to paint and other finishes
- Little effect on conductivity
- Does not increase brittleness
- No extra fumes when welding
Coating Limitations
The main limitation of any coating has to do with its properties, not its penetration throughout the steel mixture. Any worn areas of the steel will lose the benefits of the black oxide.
Apply black oxide coating using hot or cold process
We also refer to this process when people ask the question "what is black oxide". Black oxide can form in hot (285° F) or cold (room temperature or slightly higher) processes. Thermal processes have better corrosion and scratch resistance – the only type accepted by automotive and military standards.
The cold process doesn't actually create a true oxide, it just leaves a softer coating. You can scrape the coating off cold-treated steel with a coin. For products like drill bits, if the areas that come into contact with the material are cold oxidized, it doesn't take much use before the coating comes off.
Post-treatment of black oxide coatings
Since oil resists corrosion (rust), manufacturers sometimes use clear wax or even acrylic to further protect the black oxide coating. More often, they use oil after treatment. If done right, you can actually leave a fairly thick (relatively speaking) film of oil on the part. A special process exists for doing this while leaving the part dry to the touch. Most include light water soluble oils or are used with acrylic or wax after treatments.
The next time you pick up a knife or drill with a black oxide coating, hopefully you learn more about how these tools and accessories are made. If you have any additional information feel free to leave it in the comments below.